German Patent Publication No. DE 699 17 899 T2 describes using reflectance spectrometry to determine the doping of a silicon wafer. Reference and sample measurements are taken and calculated using physical calculation methods to determine the complex refractive index and the thickness of an optically thin layer. The formulas for the optical reflection of radiation are described in detail, making comprehensible the modeling of the layer to be measured. In this case, however, precise measurements can only be taken ex situ. The measurements are performed in the infrared region since the observed layers are within a thickness range that necessitates IR measurement to permit analysis of an interferometric wave to determine thickness.
A method for in situ determining the optical layer constants of layers in the context of plasma- and ion beam-assisted etching and coating is described in German Patent Application DE 197 23 729 A1. It describes a method for determining the optical constants from the reflection of light produced by the characteristic radiation of the sources used for surface finishing. During the surface finishing process, the reflection spectra are recorded from occurring interferences. Four different wavelengths are individually detected using a wavelength filter. The absolute (real) thickness of the layer is determined from the measurement signals of the normalized radiation. However, this method is only applicable to systems which use a plasma or an ion beam for surface finishing since the plasma or ion beam intensity must be additionally recorded. The substrate itself is not measured. A normalized refractive index is measured whose real value is subsequently determined. The formulas used correspond here to the Fresnel laws for a three-layer system (air—non-absorbing medium—substrate) and are limited thereto.
The European Patent Application EP 1 435 517 A1 describes using spectroscopic ellipsometers for sensing thin multilayer systems. However, ellipsometry only functions reliably in the context of very smooth layers. Moreover, an ellipsometric measurement is only possible ex situ since a polarization measurement method is needed to analyze the reflection. The polarization of the incident light is varied to allow analysis of the resultant influence on the reflected radiation. The complex refractive index is determined by varying the wavelength in a defined manner. The method is performed on multilayer systems using an optical model that is based, however, on numerically adapting (fitting) a frequency-dependent function of thickness and refractive index (similar to FFT, fast Fourier transformation).
A thickness measurement method for multilayer systems is likewise described in European Patent Application EP 1 467 177 A1. This method is based on an ex situ measurement, followed by a subsequent analysis based on a Fourier transformation. In the process, the sample is irradiated with light, and a frequency spectrum is generated by the FFT. The thickness of the individual layers is determined by analyzing the peaks of the FFT. A CCD (charge-coupled device) is used as a spatially resolving optical detector for different wavelengths.
German Patent Application DE 10 2005 023 735 A1 describes an ex situ method for automatically performing a surface examination, which provides for adapting a theoretical curve to a measuring curve using an FFT or a gradient method. In this context, however, layer thicknesses of over 10 μm are observed that are no longer to be classified as optically thin layers. The reflection spectrum is compared to a calculated spectrum. In addition, an FFT spectrum is exclusively analyzed, and the occurring peaks of this spectrum are observed. The number of layers is determined on the basis of the FFT spectrum. Thus, the layer must already be finish-processed since it would otherwise not be possible to record this type of spectrum. Different approximation methods are then derived from this spectrum.
The German Patent Application DE 10 2005 023 737 A1 describes a method for determining the layer thickness or the refractive index of a thin layer from the total reflection. It discusses determining a layer thickness or a dispersion parameter from a reflection spectrum. To this end, the measurement is compared to a model spectrum. However, only the actually changed layer is observed, the model used not being further clarified.
The aforementioned publications relate to the analysis of reflections of optical radiation in the context of smooth surfaces. All of the methods are based on the measurement of total reflection and thus require a normalization either by measuring the reference light or by subsequently measuring the refractive index using other methods. The optical models used are based on the use of Fresnel equations. No method is used to control a material vaporization process to obtain an optically thin layer.
A method employing processes for depositing chalcopyrite thin layers on moving substrates is described, for example, in German Examined Accepted Specification DE 102 56 909 B3. As a light source, a laser emitting coherent light of one wavelength is directed at a moving substrate in order to control the process of depositing and forming a chalcopyrite thin layer. In this method, the control is based on the scattering of laser light on rough surfaces. The process of vapor depositing Cu(In,Ga)Se2 layers is divided into three stages. Stage I encompasses the vaporization of indium and gallium (co-vaporization or sequential vaporization); stage II the co-vaporization of copper; and stage III the co-vaporization of indium and gallium. In addition, selenium is vaporized during the entire process. The substrate is comprehensively described in terms of material (glass, titanium or plastics) and properties. Various concepts for substrate motion are presented (pass through, rotation, roll-to-roll). Concepts are described for moving the laser that is used. Using the described method, it is possible to implement the process in a controlled manner. In the described method, the process control is based on laser light scattering (LLS) and utilizes individual characteristic points. In this context, the scatter signals of the laser light are recorded during the individual stages. In particular, the scatter signals of the second stage are utilized to estimate stoichiometric ratios in the deposited layer. However, this control does not allow a reproducible implementation of a process since this would require a feedback control with knowledge of the numerical values of the optical layer parameters. Only a qualitative monitoring of the process takes place. In the case of qualitative deviations, the production parameters “temperature of the vaporization source” and “velocity of the substrate” are varied accordingly.
In publication I by R. Scheer et al.: “Cu(In1-x,Gax)Se2 growth studies by in situ spectroscopic light scattering” (Applied Physics Letters 82 (2003), pp. 2091-2093), the LLS method is expanded to include a spectral light scattering (SLS) to be able to recognize the dependency between the roughness of the deposited layer and the scatter signal. Coherent laser light functions only in the context of rough surfaces. For smooth surfaces, as are typically present in the substrate at the beginning of the process, no analyzable measurement signals are produced. For the SLS, a white light source is used instead of a laser, and an SSD spectrometer is used as a detector. A process control based on the SLS method is likewise described in publication II by K. Sakurai et al.: “In situ diagnostic methods for thin-film fabrication: utilization of heat radiation and light scattering” (Progress in Photovoltaics: Research and Applications 12 (2004), pp. 219-234), upon which the present invention is based as the most proximate related art. However, a process control is not discussed. Here as well, the numerical values of the optical layer parameters are not known, so that the growth of thin layers cannot be quantitatively controlled, and, therefore, the actual values of the control variables (optical layer parameters) cannot be controlled to the nominal values by adjusting the manipulated variables (process parameters).
One option for quantitatively controlling the growth of thin layers is indicated, for example, in U.S. Pat. No. 5,450,250. In this method, interferometric measurements for determining the thickness of an optically thin layer are performed with the aid of a laser and a CCD camera. A laser is used to illuminate the surface of a silicon substrate. Reflections of the incident laser beam are recorded and analyzed by a CCD camera. The method is based on interferometry (compare German Patent Applications DE 10 2005 050 795 A1 and DE 10 2006 016 132 A1). In this case, incident and reflected beams are superimposed on one another in a manner that, in the extreme case, can be destructive or constructive. Absolute thicknesses of thin layers can be thereby determined A determination is only possible, however, when the optical properties of the measured material are sufficiently known. To that end, it is necessary that the magnitude of the complex refractive index be known, in particular. Moreover, it is not taken into consideration that the mentioned material properties are frequently subject to changes during the deposition process. Thus, the mentioned method provides only one fundamental possibility for quantitative process control and one possible feedback control.
For a process control in the sense of a true control loop, it is essential that the components to be controlled be rendered measurable, as is described, for example, in U.S. Pat. No. 7,033,070 B2. To monitor a crystal growth method (floating zone method) for obtaining single crystal silicon, the temperature of the molten silicon is monitored using a CCD camera. In this method, the grown crystal is locally heated by a halogen lamp. The camera is aimed at the crystal region that is heated by the halogen lamp. The CCD camera measures the luminosity of the melt, while filtering the reflected and scattered light of the light source, and thereby estimates the temperature. For this purpose, the CCD camera is equipped with filters to make the infrared light visible. Thus, the method numerically monitors silicon melts as a function of the characteristic luminosity of the melt. However, this method cannot be used to obtain information on the composition or thickness of the material. Rather, the camera monitors to what extent the crystal to be grown has melted.